A case of trial and amendment

Little information was available on composting digested sewage sludge when AFT began a two-year study to establish the optimum mix for such material. Jeff Constantine reports

The overall environmental benefits of biosolids addition to land in terms of improved soil structure and increased availability of nutrients is well documented. The development of the ADAS Safe Sludge Matrix by Water UK and the British Retail Consortium, which will be contained within the new Sludge (Use In Agriculture) Regulations, should ensure the benefits of sewage sludge can be realised whilst addressing the concerns of the government, water industry and food industry.

These regulations introduce stringent treatment requirements to ensure pathogens such as E.Coli 0157 and Salmonella spp are not transmitted into the food chain through the use of sludge as a soil conditioner when growing food crops. These sludge regulations also place a ban on the use of untreated sewage sludge on land used for growing food crops. Furthermore they introduce standards for the control and monitoring in terms of microbiological quality of sewage sludge and these standards are used to determine the restrictions of sewage sludge application to land. Two sludge microbiological treatment standards will be introduced as part of the regulations - conventional and enhanced. These will be used to qualify the level of pathogens within the sludge.

Many water companies already treat sewage sludge cake to a conventional treatment standard based on mesophilic anaerobic digestion and post-digestion sludge storage. Composting of the sludge cake has the potential to further reduce pathogens in sewage sludge to enhanced treated levels, providing sufficient heat is produced within the sludge cake material. The composting process relies on the heat released when organic carbon is oxidised to carbon dioxide by microorganisms present in the composting pile. By suitably designing and operating the composting system this heat can be used to raise the temperature of the organic mass. The elevated temperatures, which can be as high as 80°C in an uncontrolled system, are used to ensure rapid sanitisation and stabilisation. Temperatures as high as 80°C are not desirable as the organisms needed for the composting process will not function effectively at such temperatures. Although there is continuing argument about what is the 'best' composting temperature for most systems an optimum in the 55-60°C range is used.

Until now, very little work has been carried out in the UK on the composting of digested sludge to produce an enhanced treated material. The anaerobic digestion process, undertaken prior to composting the digested sludge, converts much of the readily available carbon into methane or carbon dioxide. This is the same carbon that would have formed the most readily available 'fuel' for the microorganisms carrying out the composting. Therefore the use of digested sludge in composting needs a more careful approach than with undigested sludge as there is not the same amount of carbon easily available. The critical issue for achieving good composting with digested sludge is to establish a mixture of sludge and amendment (such as greenwaste, sawdust or straw) which has the appropriate balance of nutrients and structure of the pile to ensure the smaller amount of heat generated is used to raise the temperature and not lost to atmosphere.

Controlling material

AFT has successfully achieved enhanced treatment of digested sludge using a simple outdoor composting process involving no forced aeration and minimum turning. The key to achieving this has not only been designing the appropriate mix of materials but also ensuring the practical aspects of creating a homogeneous mixture are addressed. AFT's success has been based on developing a batch-mixing machine, which provides excellent control over the addition of bulking material to achieve the correct ratios of digested sludge to amendment and also in turn produces a homogenous mix.

In the mix

During full-scale trials the two most suitable amendments to mix with the digested sludge were found to be sawdust and shredded green waste. The correct ratio of materials is crucial to achieve the appropriate balance of nutrients but also importantly to provide a suitable structure that allows air to penetrate the static pile to maintain aerobic conditions. Furthermore a homogenous initial mix of these materials is required to ensure no large lumps of sludge are generated (which results in the formation of cold spots).

After mixing using the batch-mixing machine, the material is then placed in static piles outdoors and monitored for temperature over a period of 8-12 weeks. A typical temperature profile within a pile can be found in Figure 1. This shows how the pile temperature is sustained over a period of eight weeks without any turning. This indicates also how these static piles remained predominantly aerobic even after a period of up to 12 weeks after the initial thorough mixing and homogenizing. This was the case without any forced aeration or any additional turning or mixing. This reduces the level of heat lost in the pile when compared to a frequently turned material and creates a stable temperature profile, which gives excellent pathogen reduction results. The effective homogenizing and mixture also creates any structure to the pile allowing good contact of material with oxygen.

A further benefit of not turning the piles is it dramatically reduces the amount of water vapour released helping maintain the moisture content of the pile as sufficient water is key to sustaining the microbial processes. The correct moisture content is critical to create the right conditions for composting. In most cases a moisture starting value of 60% is targeted and the material is re-wetted if the moisture content falls below 35-40% during composting, otherwise the composting process can become inhibited.

The regular turning of composting piles also leads to a release of bioaerosols to the atmosphere. The levels of odour produced during composting are significantly less due to the reduced mixing and turning. Additionally the composting of digested sludge over raw sludge significantly reduces the odour generation. The full-scale composting trials undertaken with digested sludge mixed by the AFT batch mixing system with various amendments achieved enhanced sludge standards in less than six weeks without any turning. The levels of pathogens in sludge and compost are generally assessed using indicators such as E.coli which is normally present in the digested sludge at levels around 105-106 colony forming units (cfu)/g. Table 1 shows the change in E.coli levels in sludge over a nine-week composting period.

In most of the standards used for sanitized compost an E.coli level of less than 1,000cfu/g is used. In the trials carried out by AFT with digested sludge these levels were generally achieved within four weeks which gives a considerable factor of safety as the typical composting times used are generally in excess of eight weeks



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